Fluid pump assemblies are well known in the art, and have various applications. One such application is to apply a liquid coating to an article, such that after application the liquid hardens and forms a protective or aesthetic layer on top of the article. Commonly paint is applied to an article in this manner through use of a brush or spray gun. Fluid pump assemblies provide the necessary pressure for the liquid to be sprayed, or otherwise moved through a system.
One such fluid pump assembly is disclosed in U.S. Pat. No. 6,212,997 B1, the disclosure of which is fully incorporated herein by reference. The pump described there includes a piston reciprocated in response to air pressure introduced through an air valve. The air valve operates to alternate delivery of pressurized air, to push the piston and an attached driving rod either up or down. A driven rod is connected at one end to the driving rod and at the other end to a plunger located within a hydraulic housing. The plunger incorporates an internal bore with a pressure ball check near the end which is disposed in the hydraulic housing. As the piston reciprocates, the plunger does also. Through operation of the pressure ball check, the reciprocation forces fluid into and out of the internal bore, under pressure, to be applied elsewhere. In this pump the driven rod is connected to the driving rod by a threaded connection.
Another means for connecting a driving shaft to a driven shaft is disclosed in U.S. Pat. No. 6,164,188. The coupling there comprises a two piece clamp with a recess which receives flanged ends on two reciprocating shafts. What is needed in the art is a reciprocating shaft connection assembly which holds the driving shaft securely against the driven shaft and also ensures the longitudinal axes of the two shafts remain colinear. Such a structure is not present in the prior art.
The driven shaft must be reciprocated by some mechanism, such as a piston reciprocated by air pressure. Known fluid pump assemblies, such as the one illustrated in U.S. Pat. No. 6,212,997 B1, incorporate an over-center spring switch S to switch an air valve between “up” and “down” air pressure configurations. FIG. 1 shows such a spring switch S in more detail. In these assemblies a shift arm is fixed at one end to the driving shaft and at the other end slidably fits over a push rod R, as shown in U.S. Pat. No. 6,212,997 B1. Near the end of a stroke the shift arm encounters a stop on the push rod R, causing the push rod R to move. As shown in FIG. 1, one of two rings R1 and R2 on the push rod R proximate to the air valve V is thereby caused to contact a spring switch S extension E. The spring switch S is thus moved between an “up” and “down” position.
The spring force applied by the over-center spring switch S normally is quite high to achieve fast shifting. Fast shifting is desired because, as the piston changes direction, inevitably there is a short time where the piston is not moving at all. A high spring force keeps this time to a minimum, so that the fluid is pumped at as constant a rate as possible. Otherwise spurting of the fluid being pumped (or “wink”) can occur, which is undesired in the application of liquid coatings to articles. The high spring force leads to wear of the parts, especially at the pivot points, and therefore there is a need for part replacement over time. It also can require hardened steel components, and several interconnected parts, both of which increase the cost of the system. Also, a cover is required for the push rod R and spring switch S, to protect against operators being injured by these parts during use.
It is desired therefore desired to provide a fluid pump assembly having a coupling between driving and driven shafts which permits quick connection and disconnection, and also ensures a securely maintained, axially aligned connection between the shafts. Specifically, a need exists for a single shaft coupling mechanism which, by itself, both longitudinally compresses the two shafts together and keeps their longitudinal axes aligned.
It is further desired to provide a fast-shifting switching mechanism which requires fewer wear parts, is easy to maintain and eliminates pinch points to improve operator safety. It is further desired that such a switching mechanism be manufacturable with inexpensive parts.